1. Field of the Invention
This invention is in the field of liquid processing and more particularly relates to the concentration of feed streams.
2. Description of the Prior Art
Freeze-crystallization concentration systems are used to separate liquid feed streams into more purified liquid and a concentrate. These systems have many uses among which are: conversion of a contaminated waste water stream into fresh water and concentrate; desalination of sea water; concentration of solutions or suspensions containing food such as orange juice or coffee; separation from solution of chemicals having different freezing points; etc. In some of these cases, then, the desired product is purified water which can be obtained by melting ice formed in such systems, but in other cases the desired product is the concentrate.
Freeze-crystallization concentration processes operate by taking advantage of the scientific principle that ice crystals, as they freeze, exclude dissolved impurities, including organics, inorganics and volatiles. Thus, the resulting ice crystals consist of purified water.
Generally, freeze-crystallization systems include at least one freezer-crystallizer to form ice crystals and a concentrate and at least one wash column to separate ice from concentrate and to wash the surfaces of the ice crystals. Systems which include only one freezer-crystallizer and one wash column are referred to as one-step systems.
Those systems having two freezer-crystallizers and two wash columns are referred to as two-stage systems. One example of a two-stage freeze-crystallization system is described in U.S. Pat. No. 3,885,399 granted to Robert J. Campbell. The system described in this patent recycles concentrate from the first-stage wash column and uses the recycled concentrate as wash water in the second-stage wash column. This reduces the temperature difference in the second wash column to eliminate freezing on the screens which was a serious problem encountered in two-stage systems because of the large freezing point depression produced in concentrated solutions.
Two-stage freeze-crystallization concentration systems, such as disclosed in the Campbell patent, have proven to be very successful, in general. Nevertheless, they have been limited in their ability to treat highly contaminated feed streams. One reason for this is that when the concentration of the feed stream is relatively high, the ice crystals produced are relatively small and therefore difficult to wash. The rate of ice crystal growth is related inversely to the concentration of the liquid surrounding it. In order to overcome this difficulty, one of three approaches is available: (1) dilute the feed so relatively large crystals can be harvested from the first freezer, (2) retention time in the crystallizer could be increased by a very large amount, to several hours rather than a few minutes, to enable growing of crystals of sufficient size for easy washing, and (3) extremely large wash columns can be built in which small crystals can be washed. The first of these involves excessive energy consumption, while the latter two involve high capital costs. Because of this, it has heretofore been customary to dilute concentrated feed streams prior to their introduction into the first-stage freezer-crystallizer.